MKDS is the king, but the best car racing game id have to say is NFS carbon, then ridge racer.. I thouhgt ashfalt GT's were crap. Diddy kong racing isnt too bad for some fun, but it wont keep you entertained for too long... Unless your interested in being #1 Time trialer in the world...
Apart from that, there isnt really a whole lot in the racing department thats worth while. Oh, I didnt mind NFSU2 either, but alot of people dont like it for some reason.. And dont get me started on NFS most wanted. That game is hidiouse.